During the catalytic methylation of phenol, the o-isomers frequently occur in a predominant proportion while the other isomers are obtained in a smaller quantity. However, a higher production of m-cresol may be desired and processes have therefore been developed for the isomerisation of the o-compounds into the corresponding m-compound. These processes are generally carried out in the presence of aluminium oxides (German Patent application No. 19 56 383) or aluminium silicates (German Patent application No. 20 05 153) as catalysts, but then disproportionation which is generally undesirable invariably occurs to a considerable extent in addition to isomerisation.